Field of the Invention
The present invention concerns a method for recording magnetic resonance (MR) signals from an examination object in an MR system, wherein at least part of the examination object moves during the data acquisition. The invention also concerns an MR system for implementing such a method.
Description of the Prior Art
If the examination object moves during recording (acquisition) of the MR signals, this leads to an incorrect registering of the measured MR data and to image artifacts, also called ghosting. The movement of the examination object, e.g. an examination person, usually occurs due to breathing or due to the movement of the heart or due to other undesirable movements such as a tremor in elderly or ill patients. Most movement in an examination object is periodic, wherein each cycle of movement can constitute a problem, particularly in the case of long recording times. One possibility for imaging moving examination objects is known as the single-shot technique, in which the raw data domain (k-space) is filled completely after radiation of an RF pulse and in which recording of the data is quick enough to freeze the movement. A further recording possibility is the segmented recording technique in which the data acquisition is divided into multiple movement phases and MR data are recorded only in comparable movement phases. The first possibility mentioned above limits the measurement time, and the second possibility increases the complexity of the measurement and is not effective since MR signals are recorded only in certain movement phases.
There is also the possibility of detecting the movement of the examination object and, as in the segmented recording technique, of using only the movement phases in which a slight movement occurs, but this lengthens the recording time, and this in turn increases the probability of the examination person moving. A method for recording MR signals is therefore needed in which MR data are recorded in all movement phases of a moving examination object.
One possibility for correcting movement artifacts is to fill raw data space with raw data lines such that the movement occurs when the peripheral raw data are recorded, i.e. the outer k-space region is filled, with the MR data falsified by movement then being replaced. However, this leads to a reduction in the spatial resolution due to missing portions of raw data.
Compressed Sensing Technology also is known, in which severe under-sampling of raw data spaces occurs and additional information e.g. about the recorded MR image, is used. The raw data disrupted by movement are replaced by calculated portions of raw data. Mehdi et al describe in “Compressed Sensing Motion Compensation (CosMo): A Joint Prospective-Retrospective Respiratory Navigator for Coronary MRI”, Magnetic Resonance in Medicine 66:1674-1681 (2011) the use of a method of this kind in which raw data falsified by movement are replaced with the use of Compressed Sensing Technology. A prerequisite for this is a random distribution of the remaining non-rejected MR raw data in the MR raw data space.
WO 2013/140276 A1 describes a reconstruction method in which MR raw data are recorded, wherein the density of the recorded raw data is increased incrementally until the examination person starts to breathe.
To summarize, all of the above-described methods have certain drawbacks.